High power-density, high back emf permanent magnet machine and method of making same

ABSTRACT

An electric drive system includes a permanent magnet machine having a rotor and a stator and a power converter electrically coupled to the permanent magnet machine and configured to convert a DC link voltage to an AC output voltage to drive the permanent magnet machine. The power converter includes a plurality of silicon carbide switching devices having a voltage rating that exceeds a peak line-to-line back electromotive force of the permanent magnet machine at a maximum speed of the permanent magnet machine.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims priority to,U.S. patent application Ser. No. 12/949,925, filed Nov. 19, 2010, thedisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to permanent magnetmachines having high power-density and, more particularly, to a methodand system for preventing fault conditions in a high power-density, highback electromotive force (emf) permanent magnet machines by providingpower converters that include silicon carbide metal-oxide-semiconductorfield effect transistors (MO SFETs).

The need for high power density and high efficiency electric machines(i.e., electric motors and generators) has long been prevalent for avariety of applications, particularly for hybrid and/or electric vehicletraction applications. Due to energy supply and environmental reasons,there has been increased motivation to produce hybrid-electric and/orelectric vehicles that are both highly efficient and reliable, yetreasonably priced for the average consumer. However, the drive motortechnology available for hybrid-electric and electric vehicles hasgenerally been cost-prohibitive, thereby reducing one (or both) ofconsumer affordability or manufacturer profitability.

Most commercially available hybrid-electric and electric vehicles relyon internal permanent magnet (IPM) electric machines for tractionapplications, as IPM machines have been found to have high power densityand high efficiency over a wide speed range, and are also easilypackaged in front-wheel-drive vehicles. However, in order to obtain suchhigh power density, IPM machines must use expensive sintered highenergy-product magnets. Furthermore, IPM machines run at high speed(e.g., 14,000 rpm) to obtain optimum power density. The power density ofa permanent magnet machine is defined as the ratio of the power outputand the volume of the permanent magnet machine. A relatively high powerdensity (e.g., high power output relative to volume) is typicallydesirable. The high power density allows the permanent magnet machine tohave either a smaller overall size for a given power output or a higheroutput for a given size.

As the speed of the rotor of the permanent magnet machine increases, thevoltage developed in the stator (referred to as the “back emf”)increases. This, in turn, requires that higher and higher terminalvoltages be applied to produce the desired torque. The machine back emfis proportional to speed for a permanent magnet machine. If the peakline-to-line back emf at maximum speed is higher than the DC linkvoltage, and if control over the power converter is lost, the permanentmagnet machine will start operating in an uncontrolled generation (UCG)mode. UCG occurs when the control gate signals to all of the sixinverter switches are turned off, or disconnected. During thiscondition, the motor is connected to the DC source via the anti-paralleldiodes of the inverter switches. The anti-parallel diodes create apotential path for current to flow, which is dependent upon the motoroperating condition and DC source voltage. In this case, the permanentmagnet machine will act as a generator converting rotational power intoelectric currents and will start dumping energy into the DC link throughthe anti-parallel diodes in the power converter, causing an increase inthe DC link voltage. If this energy is not dissipated, or if thebuild-up of the DC link voltage is not limited, the voltage rating ofthe active devices in the power converter may be exceeded by the DC linkvoltage.

In order to minimize or prevent occurrences of the UCG mode ofoperation, a limit is typically set on the machine back emf or anadditional clamping or crowbar circuit is added in parallel to the DClink. However, limiting the machine back emf reduces the power or torquedensity and speed capacity of the machine. Further, adding a crowbarcircuit adds additional cost and complexity to the circuitry of thepermanent magnet machine drive system. The back emf of a machine canalso be reduced by limiting the amount or relative strength of themagnets in the machine, which also negatively impacts the power ortorque density.

It would therefore be desirable to eliminate setting a machine back emflimit and/or to eliminate adding a crowbar circuit such that devicevoltage ratings are not exceeded during a UCG mode of operation.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one aspect of the invention, an electric drive systemincludes a permanent magnet machine having a rotor and a stator and apower converter electrically coupled to the permanent magnet machine andconfigured to convert a DC link voltage to an AC output voltage to drivethe permanent magnet machine. The power converter includes a pluralityof SiC switching devices having a voltage rating that exceeds a peakline-to-line back emf of the permanent magnet machine at a maximum speedof the permanent magnet machine.

In accordance with another aspect of the invention, a method ofmanufacturing an electric drive system includes the step of providing aSiC power converter that has a plurality of SiC switching devices and iscoupleable to a power source. The method also includes the steps ofproviding a permanent magnet machine having a peak line-to-line back emfat maximum speed that is greater than a DC link voltage of the powersource and coupling the SiC power converter to the permanent magnetmachine to drive the permanent magnet machine.

In accordance with another aspect of the invention, a vehicle drivesystem includes a motor that has a permanent magnet rotor and a stator.The drive system also includes a DC link and a power converterelectrically coupled between the DC link and the permanent magnet motorto drive the permanent magnet motor. The power converter comprises aplurality of SiC switching devices rated for a higher operating voltagethan a maximum back emf capable of being developed in the stator of thepermanent magnet motor.

Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate embodiments presently contemplated for carryingout the invention.

In the drawings:

FIG. 1 illustrates a conventional permanent magnet machine drive system.

FIG. 2 illustrates a high-power density permanent magnet machine drivesystem, according to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a conventional three-phase permanent magnet machinedrive system 10. System 10 includes a DC link 12 that provides a DCinput voltage that is converted or inverted to an AC waveform thatpowers a permanent magnet machine 14. An input filter capacitor 16 iscoupled across the DC link 12 for filtering the voltage V_(AC) on the DClink 12. A power converter 18 receives the input voltage from DC link 12when power flows from the DC link 12 to the AC permanent magnet machine14. This direction of power flow is often referred to operating in a“motoring” mode. When the direction of power flow is from the permanentmagnet machine 14 to the power converter 18, the input voltage to thepower converter 18 is AC from the permanent magnet machine 14, while theoutput from the power converter 18 is a DC voltage on the DC link 12.Operation with power flow from the AC permanent magnet machine 14 to thepower converter 18 is often referred to operation in a regenerativebraking mode that is useful, for example, in a vehicle where it isdesirable to hold a given value of speed on a downhill grade, or whiledecelerating the vehicle.

Power converter 18 is a typical 3-phase inverter having twoseries-connected switching devices per phase leg. For example, devices20 and 22 form a first phase leg, devices 24 and 26 form a second phaseleg, and devices 28 and 30 form a third phase leg. Devices 20-30 areconventional silicon semiconductor switching devices such as, forexample, silicon IGBT, MOSFET, silicon bi-polar Darlington powertransistor, GTO, SCR, or IGCT type devices. Diodes 32, 34, 36, 38, 40,42 are coupled in anti-parallel relationship across respective siliconswitching devices 20-30.

FIG. 2 illustrates a permanent magnet machine drive system 44 inaccordance with an embodiment of the invention. Drive system 44 includesa DC link 46 having a DC source voltage V_(S) 48. Drive system 44includes a power source 50 that provides DC source voltage V_(S) 48.Drive system 44 includes preferably two contactors (C1, C2) 52, 54, orat least one contactor C1 to couple or disconnect DC link 46 from powersource 50. In one embodiment, power source 50 includes an AC source 58and a rectifier 56 configured to convert a voltage of AC source 58 tothe DC link or source voltage V_(S). In another embodiment, power source50 includes a DC power source 58, such as a battery, a fuel cell, or aflywheel with associated power electronic converter. In yet anotherembodiment, power source 50 includes a DC power source 58, such as abattery, a fuel cell, an ultracapacitor, or a flywheel with anassociated power electronic control coupled to a bi-directional DC-to-DCvoltage converter 56 that boosts the source voltage to the DC link orsource voltage V_(S). DC link 46 supplies a DC output voltage V_(DC) 60to a power converter or inverter 62. An input filter capacitor 64 isillustrated between a positive DC rail 66 and a negative DC rail 68 andserves to provide a filter function for the high frequency currents fromsource 50 to ensure the power quality between positive and negativerails 66, 68.

Power converter 62 receives DC input voltage V_(DC) 60 from DC link 46and converts the DC input voltage to provide a suitable form of AC powerfor driving a permanent magnet machine 70, described in detail below. Acontroller 72 is also included in drive system 44 and includes means toopen and close contactors Cl and C2 52, 54 based on sensed voltageinputs from V_(S) 48, V_(DC) 60, speed sensor inputs from machine 70,plus operator inputs as well as detected faults that may occur in powerconverter 62. Controller 72 also includes means to control the boostpower command to the bi-directional boost converter 56.

According to one embodiment, power converter 62 is a three-phase DC toAC inverter having a plurality of switching devices 74, 76, 78, 80, 82,84. Each switching device 74-84 includes a silicon carbide (SiC) MOSFET86, 88, 90, 92, 94, 96 and an associated anti-parallel diode 98, 100,102, 104, 106, 108.

SiC is a crystalline substance that has material properties that make itan attractive alternative to silicon for high voltage, and high powerapplications. For example, SiC has a large bandgap that provides a verylow leakage current, which facilitates elevated temperature operation.In fact, semiconductor devices manufactured on a SiC substrate canwithstand temperatures in excess of 200 degrees C. SiC also has a highbreakdown field that is about ten times that of silicon and a thermalconductivity that is about three times that of silicon, allowing higherpower densities to be accommodated with SiC circuits. Further, SiC'shigh electron mobility enables high-speed switching. Thus, SiC has beenconsidered as an advantageous material for use in the manufacture ofnext generation power semiconductor devices. Such devices include, forexample, Schottky diodes, thyristors, and MOSFETs.

Moving from left to right in FIG. 2, switching devices 74, 76 areassociated with a first output phase 110, switching devices 78, 80 areassociated with a second output phase 112, and switching devices 82, 84are associated with a third output phase 114. While a three-phase powerconverter and three-phase permanent magnet machine 70 are illustrated inFIG. 2, one skilled in the art will understand that embodiments of thepresent invention are equally applicable to a single-phase or othermulti-phase embodiments. For example, alternate embodiments includeconfigurations with varying number of phases, e.g., n-phase, where n=1,2, 4, 5, 7, or higher number, where each phase of the power converterincludes a plurality of switching devices similar to devices 86, 88,each with associated anti-parallel diodes similar to diodes 98, 100.

Power converter 62 drives a permanent magnet machine 70. In oneembodiment, permanent magnet machine 70 is a traction motor thatincludes a permanent magnet rotor 116 and a stator 118, such as, forexample, a traction motor for powering an electric vehicle. Permanentmagnet rotor permanent magnet rotor 116, may be configured as a surfacemount, interior, or buried permanent magnet rotor, according to variousembodiments. In an alternate embodiment, permanent magnet machine 70 isan alternator that includes a permanent magnet rotor 116 and a stator118, such as, for example, a permanent magnet alternator coupled to aheat engine within an Auxiliary Power Unit (APU) for generatingelectrical power to aid in the operation of a hybrid-electric vehicle(HEV) or a Plug-in Hybrid-Electric Vehicle (PHEV).

The high voltage rating of SiC MOSFETs 86-96 allows permanent magnetmachine 70 to be designed with a high back emf without having to worryabout the uncontrolled generation mode, thereby significantly increasingthe power density of permanent magnet machine 70. That is, SiC MOSFETs86-96 have a voltage rating that exceeds the DC link voltage during anuncontrolled generation mode of permanent magnet machine 70.Conventional Si IGBT power modules used power converter circuits incommercially available on-road EV, HEV, and PHEV typically have avoltage rating of approximately 600 V or 1,200 V for some larger or highperformance vehicles, including SUV's, trucks, and buses. According toone embodiment, SiC MOSFETs 86-96 are high voltage SiC MOSFETsmanufactured by General Electric Company having a voltage rating ofapproximately three to four kV. The combined high voltage SiC powerconverter 62 combined with high power density multi-phase permanentmagnet machine 70, allows upwards of two-to-four times power densitywith a substantial improvement in fault tolerance during periods of lossof control over the power converter 62 or loss of gate drive to thepower modules within the power converter 62. Because SiC MOSFETs 86-96can be manufactured to be physically smaller than conventional siliconMOSFETs, SiC MOSFETs 86-96 can be packaged in an automotive environmentand can be operated at higher temperatures.

Excessive emf voltage of permanent magnet machine 70 may damage DC powersource 58 of power source 50. Accordingly, in one embodiment, controller72 is configured to detect a fault in power converter 62 and theassociated gate drive circuitry of power converter 62. For example, afault may be detected if the line-to-line back emf is within a thresholdpercentage of the voltage rating of DC power source 58. If a fault isdetected, controller 72 may be programmed to disconnect or decouple DCpower source 58 from power converter 62. Accordingly, excessive emfvoltage created by permanent magnet machine 70 during a fault conditionwithin power converter 62 will not result in overvoltage damage to DCpower source 58. The high voltage rating of SiC power converter 62 andits associated components 86-96 will withstand the back emf from thehigh-power permanent magnet machine 70, even if a potential fault occurswhile machine 70 is operating at high speed.

Therefore, according to one embodiment of the invention, an electricdrive system includes a permanent magnet machine having a rotor and astator and a power converter electrically coupled to the permanentmagnet machine and configured to convert a DC link voltage to an ACoutput voltage to drive the permanent magnet machine. The powerconverter includes a plurality of SiC switching devices having a voltagerating that exceeds a peak line-to-line back emf of the permanent magnetmachine at a maximum speed of the permanent magnet machine.

According to another embodiment of the invention, a method ofmanufacturing an electric drive system includes the step of providing aSiC power converter that has a plurality of SiC switching devices and iscoupleable to a power source. The method also includes the steps ofproviding a permanent magnet machine having a peak line-to-line back emfat maximum speed that is greater than a DC link voltage of the powersource and coupling the SiC power converter to the permanent magnetmachine to drive the permanent magnet machine.

According to yet another embodiment of the invention, a vehicle drivesystem includes a motor that has a permanent magnet rotor and a stator.The drive system also includes a DC link and a power converterelectrically coupled between the DC link and the permanent magnet motorto drive the permanent magnet motor. The power converter comprises aplurality of SiC switching devices rated for a higher operating voltagethan a maximum back emf capable of being developed in the stator of thepermanent magnet motor.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. An electric drive system comprising: a DC link; apermanent magnet machine comprising a rotor and a stator; and a powerconverter coupled between the permanent magnet machine and the DC link,the power converter comprising a plurality of silicon carbide (SiC)switching devices having a voltage rating that exceeds a voltage of theDC link when the permanent magnet machine operates in an uncontrolledgeneration mode.
 2. The electric drive system of claim 1 wherein thevoltage rating of the plurality of SiC switching devices is at leastthree kV.
 3. The electric drive system of claim 1 wherein the pluralityof SiC switching devices comprise SiC MOSFETs.
 4. The electric drivesystem of claim 1 further comprising: a power source coupled to the DClink; and a controller programmed to disconnect the power source fromthe DC link based on a detected line-to-line back emf of the permanentmagnet machine.
 5. The electric drive system of claim 1 furthercomprising: a DC power source; a bi-directional DC-to-DC power converterconfigured to boost an output voltage of the DC power source and supplythe boosted output voltage to the DC link; and an input filter capacitorcoupled between a positive DC rail and a negative DC rail of the DClink.
 6. The electric drive system of claim 1 wherein the powerconverter comprises a multi-phase power converter.
 7. The electric drivesystem of claim 1 wherein the permanent magnet machine comprises atraction motor.
 8. A vehicle drive system comprising: an alternator; anda power converter coupled to the alternator, the power convertercomprising a plurality of gate driven silicon carbide (SiC) switchingdevices having a voltage rating that exceeds a peak line-to-line backelectromotive force (emf) of the alternator.
 9. The vehicle drive systemof claim 8 wherein the peak line-to-line back emf of the alternator isdefined at a maximum speed of the alternator.
 10. The vehicle drivesystem of claim 8 further comprising a power source coupled to the tothe power converter through a DC link.
 11. The vehicle drive system ofclaim 10 further comprising at least contactor configured to disconnectthe power source from the DC link.
 12. The vehicle drive system of claim11 wherein the at least one contactor comprises a first contactorpositioned on a positive rail of the DC link and a second contactorpositioned on a negative rail of the DC link.
 13. The vehicle drivesystem of claim 10 further comprising a controller programmed todisconnect the power source from the DC link if a detected line-to-lineback emf of the alternator is within a threshold percentage of thevoltage rating of the power source.
 14. The vehicle drive system ofclaim 10 wherein the power source is a DC power source.
 15. The vehicledrive system of claim 14 further comprising a bi-directional DC-to-DCvoltage converter coupled between the DC power source and the DC link.16. The vehicle drive system of claim 8 wherein the alternator iscoupled to a heat engine.
 17. The vehicle drive system of claim 8wherein the plurality of gate driven SiC switching devices comprise SiCMOSFETs.
 18. A power converter for use in a drive system of a hybridelectric vehicle, the power converter comprising a plurality of siliconcarbide (SiC) switching devices having a voltage rating of at leastthree kV.
 19. The power converter of claim 18 wherein the SiC switchingdevices comprise SiC MOSFETs.
 20. A power converter for use in a drivesystem of a hybrid electric vehicle, the power converter comprising aplurality of switching devices, each of the plurality of switchingdevices consisting of a silicon carbide (SiC) MOSFET and ananti-parallel diode.